The present invention pertains to novel, acrylic-based, fumaric-acid-ester interpolymers useful as pressure-sensitive adhesives in which, surprisingly, peel and tack values of the adhesive can be modified without affecting cohesive strength, as measured as shear.
U.S. Pat. No. 2,544,692 to Kuegler et al discloses copolymerization of an acrylic ester with minor amounts of a fumarate di-ester. Emulsion and bulk polymerization are stated as being feasible.
U.S. Pat. No. 2,642,414 to Bauer et al discloses preparation of copolymers of any ester of methacrylic acid and a saturated aliphatic monohydric alcohol and a maleinoid ester of a saturated aliphatic monohydric alcohol of from 4 to 14 carbon atoms, and certain organic acids. The maleinoid esters include dibutyl fumarate. The polymers prepared were used to modify the pour point of hydrocarbon oils.
Adhesives for hydrophobic polymers containing an interpolymerized amount of a vinyl acetate, a mono- or di-alkyl ester of an alpha, beta ethylenically-unsaturated dicarboxylic acid (e.g. dibutyl fumarate) or mixture thereof, are disclosed in U.S. Pat. No. 3,275,589 to Alexander et al as being prepared by an emulsion polymerization process.
In the pressure-sensitive adhesive art, cohesive strength is measured by shear or shear strength, which is the resistance of adhesive joints to shear stresses, and is measured as the force per unit area sheared at failure. Shear is parallel to the adhesive joint.
Peel or peel strength is measured by drawing an adhesive applied to a substrate at some angle to the substrate, usually 90.degree. or 180.degree.. Tack is the resistance offered by the adhesive film to detachment from an adhered surface. It is a measure of stickness of an adhesive, or the ability to form an instant bond when brought into low-pressure contact with a substrate to which adhesive is to be adhered.
Up until now, it has been generally accepted in the art that cohesive strength may be sacrificed to increase peel and tack values of an adhesive.